Dissecting the microbial foodweb: determining the influence of phytoplankton derived organic matter substrates on bacterial growth efficiency

Project Rationale :
Bacterial growth plays a key role in regulating carbon and nutrients fluxes, and energy flow in marine ecosystems. The nutritional requirements of oceanic bacterioplankton are primarily met by organic matter produced in situ by the phytoplankton. This organic matter varies in terms of its chemical composition as a result of the species of phytoplankton from which it originated and also the mechanism by which it was released [e.g. 1]. Specifically, dissolved organic matter can be produced from phytoplankton by exudation, autolysis due to unfavorable conditions, or by their interaction with mortality agents such as grazers or pathogens. Data indicates the differences in organic matter composition associated with different sources influence bacterial growth efficiency, which is defined as the amount of new bacterial biomass produced per unit of organic carbon substrate assimilated. It is likely these differences are due to the bioavailability of the organic matter released [2] and also the specific nutritional needs of the bacterioplankton community present, i.e. which nutrients are limiting [3]. We seek to better understand the functioning of the oceanic microbial foodweb by examining the link between how organic matter is produced from phytoplankton and its effect on the growth efficiency and alleviation of nutrient limitation in bacterioplankton communities.

Methodology :
The study will employ a synthesis of laboratory, field and analytical components to disentangle the linkage between phytoplankton activities and interactions with bacterioplankton growth. Organic matter will be generated from a range of biogeochemically significant model organisms (diatoms, cyanobacteria, coccolithophores) and by the application of a variety of mortality pathways such as light limitation, pathogen infection and zooplankton grazing. The molecular composition of the matter will be characterized and related to its effect on the growth efficiency of bacterioplankton communities from a range of marine environments. The latter will be conducted during ocean going research cruises, provisionally targeting the Fe and C limited Southern Ocean and the N and P limited subtropical Atlantic [3]. The project will also examine the growth of key bacterioplankton species in vitro when cultivated in the presence of different phytoplankton species under various conditions, such as with a co-occurring virus. This combination of culturing and experimentation on in situ communities facilitates the examination of the ecological and biogeochemical effects of individual pathways. It is anticipated that this combination of laboratory and fieldwork, along with the arsenal of techniques available, will generate a step change in our understanding of microbial food web functioning.

Training:
The INSPIRE DTP programme provides comprehensive personal and professional development training alongside extensive opportunities for students to expand their multi-disciplinary outlook through interactions with a wide network of academic, research and industrial/policy partners. The student will be registered at the University of Southampton and hosted at the National Oceanography Centre’s Ocean Biogeochemistry and Ecosystems Research Group and the University’s Department of Ocean and Earth Sciences. The student will be primarily based in the state-of-the-art Mike Fasham Laboratory for marine microbial research which is equipped with sorting flow cytometers, laser dissecting microscope, scintillation counter, facilities for handling radioisotopes and also benefits from the recently completed culturing facility. Specific training will include: culturing of microorganisms and their associated pathogens and predators, handling of radioisotopes, bioassays and stable isotope tracing experiments, flow cytometry, approaches for the measurement of respiration, experimental design and the option for organic matter characterization.